A problem generally encountered in cooking with microwaves resides in obtaining a good distribution of the microwave energy in the cooking chamber. Thus, it is known that a stationary microwave arrangement occurs in the cooking chamber during operation of the oven. As a result, the electrical fields of the excited modes in the chamber have nodes and antinodes of voltage to which correspond respectively so-called cold and hot points at different locations in the chamber.
Numerous solutions have already been proposed to improve cooking or reheating of a product (solid or liquid food) by microwave energy.
The first type of solution consists in providing, in a cooking chamber, an agitator for the waves so as continuously to modify the stationary wave regime established in the chamber, thereby to displace the hot points and the cold points.
Another which is very widely used at present, consists in placing the product to be reheated or to be cooked on a turning plate. The relative displacement of the product and the hot points thus permits uniform cooking.
Improvements in the distribution of the microwave energy have also been obtained by supplying the cooking chamber with microwave energy by the bias of two openings provided in one of the walls of the cooking chamber. FIGS. 1 and 2 show schematically the internal portions of a known oven operating according to this principle: in these figures, there is shown a cooking chamber 1 delimited by a back wall 10, a top wall 11, a bottom wall 12 and two side walls 13 and 14. The side wall 13 has two horizontal openings 130 and 131 superposed vertically along the wall 13, for the introduction of microwave energy. The waves are generated by the antenna of a magnetron (not shown), and transmitted to the cooking chamber by means of a waveguide 2. The waveguide 2 has the general shape of a rectangular parallelepiped whose longitudinal axis is vertical. The guide is delimited transversely by two flat rectangular surfaces 20 and 21, perpendicular to the longitudinal axis of the guide and separated by a predetermined distance d defining the length 1 of the guide. These two surfaces define reference planes from which the guided waves are reflected. The two surfaces 20 and 21 are interconnected at a right angle by two rectangular surfaces 22,23, parallel to the side wall 13, each surface having two edges b.sub.1, b.sub.2, b.sub.3, b.sub.4 of a length d. The surfaces 22 farthest from the side wall 13 has a lateral prolongation 24 provided with an access opening to receive waves generated by the magnetron antenna. The surface 23 constitutes the output plane for the guided waves and comprises for this purpose two openings 230 and 231 disposed facing the openings 130 and 131. In certain known embodiments, the output plane of the waveguide is directly constituted by a portion of the side wall of the chamber.